The reasons for using the mechanical properties of soft biological tissues as a source of information for assessing the functional state of tissues and organs are the following:
The intensity of metabolic processes taking place in biological tissues depends on both the internal and external environmental influences, which bring about changes in the mechanical properties of the tissues.
Physical activity, for example, causes hypertrophy of skeletal muscles, whereas inactivity causes atrophy. As a result of various neurological diseases and traumas the muscular tone undergoes significant changes. The muscular tone is characterized by means of the stiffness and dempferity properties of muscles. The stiffness depends on the intramuscular pressure, whereas the dempferity varies according to elasticity properties of the morphological structures of muscles. The human support-motor system has developed in such a way that the rotation process of a body part round the axis of a joint always has two antagonistic muscle groups involved. The agonists create a torque in relation to the axis of the joint, which is necessary for initiating the motion, while at the same time the agonists are stretched. From the point of view of movement energetics and moving in general it is important what amount of mechanical energy is needed to stretch the antagonists. This mechanical energy in its turn comprises two parts: first, the constraining force which depends on the antagonist's tonicity, the area of its cross-section, and its change in length, and second, the resisting force, caused by the dempferity properties of the antagonist with the same change in length, which depends on the velocity of stretching. On the one hand, the tone of skeletal muscles depends on the intensity of efferent innervation, on the other, on the cellular tone. The co-influence of both can cause an increase in the mechanical tension of the envelopes or facias of the organs. The elasticity of the collagen fibres of muscle envelopes in its turn affects the ability of biological tissues to dissipate mechanical energy.
Thus changes in the mechanical properties of biological tissues can cause traumas or pathological processes. At the same time, data about changes in the mechanical properties of skeletal muscles enable us to estimate the efficiency of surgical operations, physiotherapeutic procedures, massage, rehabilitation gymnastics and drug treatment.
From what was said above we can conclude that the mechanical properties of biological tissues can yield important information about the functional ability of the tissues, thus making it possible to predict the results concurrent with changes in the mechanical properties of biological tissues.
Various devices and methods have been developed for ascertaining the mechanical properties of soft biological tissues.
The common disadvantage of the methods used until now has been that either these methods themselves cause changes to the mechanical properties of the tissue under investigation or the measuring procedure lasts so long that the subject under investigation manages to change the mechanical properties of the tissue voluntarily during the procedure. Take for instance the device for measuring muscular tone which includes two cuffs. Upon one cuff an acceleration transducer is attached, while on the other cuff the mechanical impacts are produced (Author's certificate of the USSR No. 150573, A 61B 5/05, Fedorov V. L., Talysev F. M. 1961). In this case the cuff fastening causes the amount of blood in the muscle to increase, which brings about corresponding changes in the muscular tone. The results of measurements depend on how long the cuffs have been fastened to the muscle.
From among the known methods for measuring the mechanical properties of soft biological tissues, the one most similar to the present invention is the so-called method of damping oscillations, the essence of which lies in subjecting the biological tissue under investigation to external mechanical impact and subsequently recording the mechanical response of the tissue as a graph of its damping oscillations (Fenn W. G., Garwey P. H. , J. Clin., Invest. 1934, 13, Pp. 383-397; Vajn A., Metod zatuhajuscih kolebanij pri diagnostike funkcional'nogo sostojanija skeletnyh mysc. Sb. naucnyh trudov. Metody vibracionnoj diagnostiki reologiceskih harakteristik mjagkih materialov i biologiceskih tkanej.--Gorkij, 1989. Pp. 116-125.).
Several devices have been designed and constructed with the aim of using the method of damping oscillations, however, due to inadequacies in their construction the results of measurements have been metrologically unreliable. The main shortcomings have been the unstable construction of the mechanism used for producing the mechanical impact to the biological tissue and deficiencies of the system used for recording the mechanical response of the biological tissue to the influence. The graph of the recorded damping oscillations obtained by means of these devices either included additional information about the elasticity of the parts of the device itself, or the signal of the device was too weak to provide metrologically reliable data about the characteristics of the mechanical properties of the biological tissues investigated.
A method and a device have been invented for non-invasive ultrasonic monitoring of the fluctuations of the properties of living tissue related to its viscoelasticity (U.S. Pat. No 4,580,574, A 61 B 10/00, B. Gavish, 1986). There is also another device for measuring the mechanical properties of soft biological tissues. (Author's Certificate of the USSR No. 1517939, A 61 B 5/10, Godin E. A., Cernys V. A. and Stengol'd E. S., 1988).
The common shortcomings of the above-mentioned two are that during the measuring procedure the subject under investigation can voluntarily change the muscular tone, since during the measuring procedure the subject is in a fixed permanent connection with the measuring apparatus; b) the fixed position of peripheral biological tissues in the measuring apparatus sets certain limitations to the possibilities of carrying out a test; c) repeated measurements provide a low level of accuracy.
Of all the known devices, the one closest in design to the present invention is the device for measuring biomechanical characteristics of biological tissues (Author's Certificate of the USSR No. 1782537, A 61 B 5/05, G 01 N 3/30, A. Vain, L.-H. Humal, 1992). The device consists of a gripped frame, into which a pivoting two-shouldered lever is attached. To the end of one shoulder two electromechanical transducers and a wheel-shaped testing end are fastened, and to the end of the other shoulder the drive of the testing end together with its controller. The device is equipped with an elastic element situated between the frame and the two-shouldered lever, and a recorder.
Among the deficiencies of the prototype first of all the presence of the same above-mentioned elastic element must be mentioned, which sets certain limits to the recording of such oscillation frequencies which are lower than the oscillation frequency of the above-mentioned elastic element, thus making signal processing more complicated, which reduces the accuracy of measurements. Also it is difficult to find for the elastic element of the device an elastic material with such stable mechanical properties which would last for a long exploitation period.
Secondly, in case an elastic element is used it is important to what extent the elastic element has been deformed before the mechanical impulse is produced by the drive of the testing end. In case the deformation of the elastic element as well as the force used to create the initial pressure of the testing end against the biological tissue are of a small order, and the mechanical impulse created by the drive is of a relatively great magnitude, the testing end may lose its contact with the biological tissue under investigation during the oscillation process after the switch-off of the drive of the testing end. So, as a consequence of systematic errors, the resulting graph of oscillations and its analysis will become valueless.
Thirdly, it has been established that in case the initial pressure of the elastic element is of a relatively great magnitude, then the mechanical impulse produced by the drive of the testing end can neither cause any significant deformation of the biological tissue nor evoke any noticeable damping oscillations when the drive of the testing end is switched off after the mechanical impulse has been performed. In addition to this, the creepability and relaxation properties of biological tissues will further complicate the measuring procedure and reduce its accuracy, when the initial pressure of a great magnitude is used.